Consider whether `ICompletionSource` can be replaced with a delegate or class

[jonsequitur]

    // As always, prefer classes to interfaces.
    // If you think this is really a capability interface ("ICompletable"), then perhaps it's OK; but classes are "always" preferred.
    public interface ICompletionSource 
    {
        IEnumerable<CompletionItem> GetCompletions(CompletionContext context);
    }